Installation guide

Traffic Shaping to Reduce Bottlenecks Network Requirements and Preparation
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ShoreTel 14.2 Planning and Installation Guide 51
Traffic Shaping to Reduce Bottlenecks
With many applications requiring WAN bandwidth, the need to optimize is increasingly important. This
is particularly true for enterprises that want to deploy voice over virtual networks where quality of
service and traffic shaping are required. With traffic shaping, it is possible to set policies that determine
who or what gets top priority. For example, by prioritizing the various flows of traffic, an administrator
can make sure that UDP (voice) traffic gets a higher priority than FTP (file download) traffic.
Echo Cancellation
Echo is a consideration for networks and IP phones.
Network
Echo in a voice communication system is caused by signal reflections generated by the electrical
circuits (called hybrids) that convert between two-wire (shared transmit and receive pair) and four-wire
circuits (separate transmit and receive pairs). These reflections cause the speaker’s voice to be heard
in the speaker’s ear as delayed by many milliseconds. Echo is present even in the traditional circuit-
switched telephone network, but because the delay in a local circuit-switched call is so low, the echo is
not perceivable. On a packet-based voice network, there is more delay, and the speaker may perceive
the echo if it is not properly cancelled.
The DSP software on ShoreTel voice switches provides dynamic echo cancellation. When a user
places an extension-to-trunk call using an analog trunk on a ShoreTel voice switch, the user’s voice
bounces off the initial four-wire to two-wire conversion in the analog trunk circuit, then off the two-wire
to four-wire in the central office, and finally off the called party’s telephone. This echo returns from the
central office and is cancelled by the echo canceller on the trunk port of the voice switch. The echo
from the called party’s phone, however, is usually cancelled or suppressed by the central office. If this
echo is not cancelled, users might hear themselves talking.
In the opposite direction, the external person’s voice bounces off the user’s telephone. This echo
returns from the telephone and is cancelled by the echo canceller on the telephone port of the voice
switch. If this echo is not cancelled, the external party hears himself or herself talking. This same
process of echo cancellation applies to extension-to-extension as well as trunk-to-trunk calls.
ShoreTel voice switches can cancel echo received up to 16 ms after being sent.
IP Phones
Most ShoreTel IP phones have hands-free, full-duplex speakerphones with built-in echo cancellation.
The ShoreTel IP420, however, has a half-duplex speakerphone, and for this reason it is not
appropriate for use as a speakerphone in a conference room.